More expensive bib have better fabrics and seat pads. What kind of better, well now that depends on the buyer. It's like anything really.
The Black Bibs Ultimate say this :
Now the Technical Stuff:- Longer Inseam and option for longer strap length
- Upgraded silicon-infused leg opening
- Dyed black high compression fabric
- Ultimate Pad (color varies)
- White or black mesh bib straps
I've considered buying an Ultimate bib based on the pad shape and design but I'm quite happy with the Sportful LTD bibs I wear. I think the BB Ultimate is worth every penny over the regular ones.